Methods of discovering neighbors in opportunistic open access ad hoc wireless networks

ABSTRACT

In an ad hoc wireless network, for purposes of neighbor discovery, a transmitting node transmits a beacon within a timeslot and at a frequency that varies from timeslot-to-timeslot according to one or more associated and known pseudo-random or cyclical frequency hopping sequences. When, during a timeslot, the frequency hopping sequence would select a beacon frequency that if transmitted would violate spectrum policy that is in place during that timeslot, then, during that timeslot, a beacon is not transmitted during that timeslot. During each timeslot, a neighbor receiving node attempts to detect and decode a transmitted beacon at a frequency specified by the frequency hopping sequence that it expects a transmitting node to be using. When a receiving node successfully detects and decodes a beacon transmitted by a transmitting node, neighbor discovery between the transmitting and receiving nodes is achieved.

GOVERNMENT CONTRACT

This invention was made with Government support under Contract CNS0434854 1/3 awarded by the National Science Foundation. The Governmenthas certain rights in this invention.

TECHNICAL FIELD

The present invention relates to wireless communications.

BACKGROUND OF THE INVENTION

Spectrum allocations are carried out according to prevailing regulatorypolicies in different regions throughout the world. Except for smallblocks of spectrum that have been set aside for unlicensed operation,spectrum is typically treated as property with associated ownershiprights. Since spatial and temporal usage of spectrum varies according tothe traffic demands from different services, such static allocationscreate an illusion of spectrum scarcity, i.e., the unavailability ofspectral resources at a particular location and time. Recently, therehas been considerable interest in designing Media Access Control (MAC)and physical layer techniques that allow open, opportunistic access tospectrum. This capability is desirable not only to address the long-termprojected capacity demands of commercial, military and public safetyapplications, but also to enable unlicensed commercial or tacticalmilitary communications without any frequency planning or coordination.

Physical layer techniques, protocols and algorithms employed in evolvingcellular and wireless Local Area Network technologies (e.g., IEEE802.11) are typically designed assuming a static, contiguous spectrumallocation constraint and are often limited to narrowband operation(tens of kHz to a few MHz). In the future, advances in dynamic spectrumaccess techniques such as spectrum sensing and characterization,frequency agility, and dynamic radio bearer management hold the promiseof huge improvements in spectrum utilization relative to current staticallocations by providing the ability to exploit temporal and spatialvariability in spectrum availability.

One of the outstanding challenges in ad hoc networks based onopportunistic, open spectrum access is on the design of techniques thathelp achieve initial neighbor discovery and association of new nodeswith a neighborhood. In particular, when spectrum is not exclusivelyallocated to users and/or networks, users must discover the presence ofneighboring users by searching across a wide range of frequencies withminimum transmit/receive power requirements and with minimum delay. Thisneeds to be carried out in advance of opportunistic establishment ofradio bearers for control and/or data transfer. Another challenge in adynamic spectrum access framework is to ensure that the neighbordiscovery process does not result in excessive interference related toco-existence with non-cooperative nodes (e.g. belonging to legacynetworks). Careful consideration must be given to such co-existencescenarios when designing protocols and algorithms that enable openspectrum access.

Neighbor discovery typically involves transmission of beacons (probemessages) subject to certain criteria and also scanning for beacons fromcandidate neighbor nodes. These probe messages may indicate severalparameters of interest including the address (or identifier) of thenode, location, spectral quality measurements etc. which may be used forresource allocation, routing or forwarding decisions and energyconservation. Neighbor discovery is said to occur upon successfuldetection and decoding of a probe message from a cooperative node whosepresence was previously unknown.

While it may be possible to improve neighbor discovery performance bylimiting beacon transmissions to a fixed, pre-determined region of thespectrum, such an approach is not scalable, e.g. to support openspectrum access networks with large numbers of cooperative nodes orlarge numbers of co-existing networks. Furthermore, fixed regions of thespectrum are also highly susceptible to jamming or interference fromnon-cooperative nodes, thus making it difficult for cooperative nodes todiscover each other. If cooperative nodes are not able to discover eachother, then opportunistic use of spectrum for data transfer betweenthese nodes is not possible.

The state of the art on neighbor discovery focuses on static and/orsmall allocations of spectrum. For example, Wireless Local Area networksbased on 802.11x standards use different beacon frame transmission andreception techniques depending on the mode of operation. In aninfrastructure mode, Access Points carry out periodic beacon frametransmissions on a frequency channel and all other nodes scan acrossdifferent channels to detect the presence of Access Points. In an ad hocmode, each node that is attempting discovery scans for beacons over acertain time period and transmits a beacon on a particular channel aftera random delay if none are detected. The results of a performanceanalysis of neighbor discovery for ad hoc networks with random beacontransmission and random reception is described by L. Gallulccio, G.Marbit and S. Palazzo, in “Analytical Evaluation of a Tradeoff BetweenEnergy Efficiency and Responsiveness of Neighbor Discovery inSelf-Organizing Ad Hoc Networks,” IEEE Journal on Selected Areas inCommunications, Vol. 22, No. 7, September 2004. That work was based onpre-fixed frequency carrier sets and does not apply to a dynamicspectrum access framework where frequencies deemed acceptable for beacontransmission and/or reception may vary from node to node according tothe perceived spectral quality. Furthermore, no consideration was givento policy constraints associated with opportunistic open access wirelessnetworks.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, when in astate of neighbor discovery and transmitting beacons, a transmittingnode transmits a beacon within a timeslot AND at a frequency that variesfrom timeslot-to-timeslot according to one or more associated and knownpseudo-random or cyclical frequency hopping sequences. When, during atimeslot, the frequency hopping sequence would select a beacon frequencythat if transmitted would violate an existing spectrum policy in placeduring that timeslot, then, during that timeslot, a beacon is nottransmitted. During each timeslot, a neighbor receiving node attempts todetect and decode a transmitted beacon at a frequency specified by thefrequency hopping sequence that it expects a transmitting node to beusing. When a receiving node successfully detects and decodes a beacontransmitted by a transmitting node, neighbor discovery between thetransmitting and receiving nodes is achieved.

In a Single Frequency Hopping Sequence (SFHS) embodiment, a transmittingnode uses a single frequency hopping sequence to determine thesuccessive transmission frequencies and determines for each timeslotwhether transmission during that timeslot at the frequency specified bythe hopping sequence violates spectrum policy. A beacon is nottransmitted during a timeslot at the frequency specified by the hoppingsequence if spectrum policy would be violated. The receiving nodeattempts to detect and decode during a timeslot at a frequencydetermined by that same known frequency hopping sequence.

In a Multiple Frequency Hopping Sequences with Random Sequence Selection(MFHS-RSS) embodiment, the transmitting and receiving nodesindependently randomly choose from among multiple frequency hoppingsequences for each timeslot. When, during a timeslot, the chosen hoppingsequence would result in the transmission of a beacon at a frequencythat would violate existing spectrum policy, a beacon is not transmittedduring that timeslot. During each timeslot, the receiving node randomlyselects one of the possible hopping sequences and decodes at thefrequency specified by that selected hopping sequence for that timeslot.

In a Multiple Frequency Hopping Sequences with Policy-Based SequenceSelection (MFHS-PBSS) embodiment, multiple frequency hopping sequencesare similarly allowed. Attempts are made during each timeslot, however,to maximize the probability that a transmission will be allowed perprevailing spectrum policy by pruning the possible multiple hoppingsequences used by a transmitting node to only those that would specify atransmission frequency that is allowed by spectrum policy, and thenrandomly selecting one of those hopping sequences and its associatedfrequency for transmitting the beacon during that timeslot. If none ofthe hopping sequences has an associated frequency that is allowed byspectrum policy during that timeslot, then no beacon transmission ismade during that timeslot. As in the previous embodiment, during eachtimeslot the receiving node randomly selects one of the possible hoppingsequences and attempts to decode on the frequency specified by thathopping sequence during that timeslot.

In a Multiple Frequency Hopping Sequences with Sequential EnergyDetection and Decoding (MFHS-SeEDD) embodiment, transmitting andreceiving nodes enter transmit, receive and idle discovery states inframes consisting of L timeslots. Operation at the transmitting node issimilar to the MFHS-RSS embodiment with a single hopping sequence beingused within a frame. The receiving node performs energy detection acrossthe first LED timeslots of the L-length frame to reduce the number ofcandidate hopping sequences that might have been used for transmissionof the beacon. In the last L_(DEC) timeslots of the frame, the receivingnode randomly attempts to decode based on the reduced set of candidatesequences determined during the first LED timeslots.

In a Multiple Frequency Hopping Sequences with Simultaneous EnergyDetection and Decoding (MFHS-SiEDD) embodiment, energy detection anddecoding are performed in the same timeslot, or based on informationgathered by energy detection in previous timeslots within an L-lengthframe.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram showing a neighborhood of nodes in an ad hocwireless network that might at some time be desirous of communicatingwith each other in either a one-to-one basis or one-to-many basis; and

FIG. 2 shows an example of the SFHS embodiment;

FIG. 3 is a flowchart showing the steps at a transmitting node inaccordance with the SFHS embodiment;

FIG. 4 is a flowchart showing the steps at a receiving node inaccordance with the SFHS embodiment;

FIG. 5 shows an example of the MFHS-RSS embodiment;

FIG. 6 is a flowchart showing the steps at a transmitting node inaccordance with the MFHS-RSS embodiment;

FIG. 7 is a flowchart showing the steps at a receiving node inaccordance with the MFHS-RSS embodiment;

FIG. 8 shows an example of the MFHS-PBSS embodiment;

FIG. 9 is a flowchart showing the steps at a transmitting node inaccordance with the MFHS-PBSS embodiment;

FIG. 10 shows an example of the MFHS-SeEDD embodiment;

FIG. 11 is a flowchart showing the steps at a receiving node inaccordance with the MFHS-SeEDD embodiment;

FIG. 12 shows an example of the MFHS-SiEDD embodiment; and

FIG. 13 is a flowchart showing the steps at a receiving node inaccordance with the MFHS-SiEDD embodiment;

DETAILED DESCRIPTION

Although the following is described as being based on a generic wirelesscommunication network or system supporting ad hoc communication inunlicensed spectrum, and will be described in this exemplary context, itshould be noted that the exemplary embodiments shown and describedherein are meant to be illustrative only and not limiting in any way.Additionally where used below, the term “node” may be consideredsynonymous with user equipment, terminal, mobile terminal, sensor node,subscriber, user, remote station, mobile station, access terminal, etc.,and describes a remote user of wireless resources in a wirelesscommunication network.

FIG. 1 is a block diagram of an ad hoc wireless communications system100 in which plural mobile nodes 101 are capable of communicating witheach other. Each node 101 has a transmitter for transmitting messages toone or a plurality of the other terminals. Similarly, each node 101 hasa receiver for receiving the messages sent to it by another node 101.Messages that are transmitted by a node 101 can be sent point-to-point(i.e., unicast) to another node within the transmitting node'scommunications range, can be sent to a targeted group of nodes (i.e.,multicast within the transmitting node's communications range, or can besent to all nodes within the transmitting node's communications range(i.e., broadcast). A message header may specify the message type (i.e.,unicast, multicast or broadcast), and if unicast or multicast, theintended node or group recipient. Whereas all ad hoc messages arebroadcast in nature, such that all nodes within communications range canattempt to decode the message, specification of the message type in themessage header allows nodes to increase battery life by ignoringpayloads of messages for which it is not an intended recipient. Inaddition, the payload may be encrypted.

Beacon transmissions may occur on a single frequency or may occur onmultiple frequencies that may either be contiguous or non-contiguous;for simplicity, it is assumed that the transmission occurs on a singlefrequency. Furthermore, time-slotted beacon transmissions and idleperiods where no transmission or reception occurs in order to conserveenergy are assumed. Nodes are also assumed to be time-synchronizedthrough the availability of a common system time reference at each node(e.g. via GPS capability within each node). Neighbor discovery may alsobe carried out with asynchronous nodes using timing acquisitiontechniques, but this may take longer than the synchronous case.

At any point in time, a node may be transmitting beacons, attempting toreceive beacons, or be idle (i.e., neither transmitting beacons norattempting to receive beacons). Beacons are assumed to containsufficient information (e.g. mobile ID, group ID, policy information,available protocols, etc.) so as to allow nodes to communicate with eachother after discovery is achieved. Hence, for the purposes of thisdescription herein, neighbor discovery is assumed to be achieved when anode transmits a beacon and a neighboring node receives that beacon andsuccessfully decodes it.

Various embodiments of methods for neighbor discovery are described ingreater detail below.

In a Single Frequency Hopping Sequence (SFHS) neighbor discoveryembodiment, the transmitting node uses a single cyclical orpseudo-random frequency hopping sequence to determine transmissionfrequencies during those successive timeslots when discovery is takingplace. Since the hopping sequence is pre-determined, the transmittingnode determines for each timeslot whether transmission of a beacon atthe frequency specified by the hopping sequence would violate spectrumpolicy during that timeslot. If such a transmission would violatespectrum policy, a beacon is not transmitted during that timeslot. Thereceiving node attempts to decode during a timeslot and on a frequencychannel according to the same frequency hopping sequence. Since thetransmitting and receiving nodes use the same single hopping sequence,there is no ambiguity as to the frequency at which a beacon transmissionwill occur during each timeslot. Hence, if a transmitting node transmitsa beacon, a neighboring receiving node that seeks to receive beacontransmissions is likely to receive the beacon and complete neighbordiscovery with the transmitting node provided the prevailing radiofrequency (RF) conditions are sufficient.

FIG. 2 illustrates an example of the operation of this method with twonodes attempting to discover each other as neighbors. In the generalcase, as previously noted, each node from timeslot to timeslot (or fromframe to frame where each frame consists of multi-slots) may betransmitting beacons, attempting to receive beacons, or be in an idlestate. However, for simplicity, it is assumed that when policy allowsit, a first node continuously seeks to transmit and a second nodecontinuously seeks to receive. Discovery occurs when the second nodesuccessfully receives a beacon that is being transmitted by the firstnode. In FIG. 2, the frequency hopping (FH) sequence specifies f₃ as thebeacon frequency during the first two timeslots, t₀ and t₁. In thisillustrative example, transmission during these first two timeslots atf₃ is disallowed by spectrum policy, as noted in the figure, and thusdoes not occur. During the third timeslot, t₃, the frequency specifiedby the frequency hopping sequence is f₇, which is not disallowed byspectrum policy, and the transmitting node is thus allowed to transmit abeacon at that frequency. Since there is a single frequency hoppingsequence utilized by both the transmitting node and receiving node,neighbor discovery is achieved during this third timeslot. It should benoted that a spectrum policy that governs the use of frequencies,perhaps at different points in time, might be identical or somewhatdifferent between the transmitting node and receiving node.

The flowchart in FIG. 3 shows the steps at a transmitting node accordingto this embodiment. At step 301, the transmission frequency of a beaconis determined for a timeslot according to a pseudo-random or cyclichopping sequence. At step 302, a determination is made whethertransmission of a beacon at that frequency would violate spectrum policyduring that timeslot. If it would violate spectrum policy, then, at step303, no beacon is transmitted during that timeslot. If spectrum policywould not be violated, then, at step 304, a beacon at that determinedfrequency is transmitted during that timeslot.

The flowchart in FIG. 4 shows the corresponding steps at a receivingnode according to this embodiment. At step 401, for a timeslot, thefrequency at which beacon detection will be attempted is determinedaccording to a known pseudo-random or cyclic frequency hopping sequence.At step 402, a determination is made whether spectrum policy is violatedduring that timeslot at that frequency. If spectrum policy is notviolated, then, at step 403, no attempt to detect and decode a beacon atthat frequency is made during that timeslot. If spectrum policy is notviolated, then, at step 404, at that timeslot an attempt is made todetect and decode a beacon at that frequency and thus, discovery is notachieved. At step 405, if RF conditions are sufficient, the receivingnode successfully detects and decodes the beacon thereby achievingneighbor discovery between the transmitting node and the receiving node.

In a Multiple Frequency Hopping Sequences with Random Sequence Selection(MFHS-RSS) embodiment, the transmitting and receiving nodes are allowedto choose from among multiple frequency hopping sequences. In thisembodiment, it is assumed that a transmitting node randomly selects oneof B_(n) hopping sequences. As per the above-described SFHS method, atransmitting node only transmits a beacon when it is allowed by spectrumpolicy. For each timeslot, the receiving node randomly selects one ofB_(n) hopping sequences and attempts to decode on the frequencyspecified by that hopping sequence. Thus, the probability that discoverywill occur in a particular timeslot is scaled by (1/B_(n)) relative tothe SFHS method.

Using the same assumptions described in the SFHS method above, FIG. 5illustrates an example of the operation of the MFHS-RSS method. In thisexample embodiment, there are two frequency-hopping sequences. For thefirst timeslot, t₀, FH sequence 0 specifies f1 and FH sequence 1specifies f₅. The transmitting node randomly selects FH sequence 0 andtransmits at f₁ while the receiving node randomly selects FH sequence 1and attempts to decode at f₅. Neighbor discovery is thus not achievedduring timeslot to. For the second timeslot, t₁, the frequenciesspecified by both FH sequences 0 and 1, f₃ and f₇, respectively, aredisallowed by spectrum policy and neither is transmitted. For the thirdtimeslot, t₃, the transmitting node randomly selects FH sequence 0, butthe specified frequency, f₄, for this FH sequence is disallowed byspectrum policy, so no transmission is made. For the fourth timeslot,t₃, both transmitting and receiving nodes select the same hoppingsequence, FH sequence 0, and discovery is achieved when the transmittingnode transmits a beacon at f₆ and the receiving node detects and decodesthe beacon on that same frequency.

The flowchart in FIG. 6 shows the steps at a transmitting node inaccordance with the MFHS-RSS embodiment. At step 601, for a timeslot,one of B_(n) possible hopping sequences is randomly selected. At step602, for that timeslot and using the selected hopping sequence, thebeacon frequency is determined. At step 603, a determination is madewhether transmission of a beacon at the determined frequency during thattimeslot would violate spectrum policy. If spectrum policy would beviolated, then, at step 604, a beacon is not transmitted during thattimeslot. If it does not violate spectrum policy, then, at step 605, abeacon is transmitted at the determined frequency during that timeslot.

The flowchart in FIG. 7 shows the steps at a receiving node inaccordance with the MFHS-RSS embodiment. At step 701, for a timeslot,the receiving node randomly or cyclically selects one of the B_(n)hopping sequences for decoding. At step 702, for the selected hoppingsequence, the frequency is determined at which detecting and decodingwill be attempted during that timeslot. At step 703, a determination ismade whether spectrum policy is violated at that frequency during thattimeslot. If violated by spectrum policy, then, at step 704, no attemptis made to detect and decode at that frequency during that timeslot. Ifspectrum policy is not violated, then, at step 705, during thattimeslot, an attempt is made to detect and decode at the frequencyspecified by the selected hopping sequence. At step 706, a determinationis made whether that specified frequency is the frequency at which thetransmitting node is actually transmitting a beacon (i.e., the receivingnode has correctly selected the hopping sequence being used in thattimeslot by the transmitting node). If it is and RF conditions are good,then at step 707, neighbor discovery between the transmitting andreceiving nodes is achieved. If it is not the frequency at which thetransmitting node is actually transmitting a beacon, at step 708,neighbor discovery is not achieved during that timeslot. Successfulneighbor discover awaits another subsequent timeslot when both thetransmitting and receiving nodes are transmitting and receiving,respectively, at the same frequency.

The Multiple Frequency Hopping Sequences with Policy Based SequenceSelection (MFHS-PBSS) embodiment is similar to the above-describedMFHS-RSS embodiment in that it allows B_(n) frequency hopping sequences.Unlike the MFHS-RSS embodiment, however, attempts are made to maximizethe probability that transmissions are allowed as per prevailingspectrum policy. In particular, for each timeslot, the transmitting nodeprunes the set of possible B_(n) hopping sequences by eliminating anyhopping sequence whose frequency during that timeslot violates spectrumpolicy. A frequency for transmitting the beacon during a timeslot isthen randomly selected from among the frequencies associated with theremaining subset of allowed candidate hopping sequences. If none of thecandidate hopping sequences has during that timeslot an associatedfrequency that is allowed by spectrum policy, no beacon transmission ismade. Similar to the previous MFHS-RSS embodiment, for each timeslot,the receiving node randomly selects one of the B_(n) hopping sequencesand decodes on the frequency specified by that sequence.

FIG. 8 illustrates an example of this MFHS-PBSS embodiment. For thefirst timeslot, t₀, the transmitting node randomly selects FH sequence 0and transmits on f₁ and the receiving node randomly selects FH sequence1 and attempts to decode on f₅ so that discovery is not achieved. Forthe second timeslot, t₁, the frequencies, f₃ and f₇, specified by FHsequence 0 and FH sequence 1, respectively, are both disallowed bypolicy and no beacon is transmitted. For the third timeslot, t₂, thefrequency f₄, specified by FH sequence 0 is disallowed by policy but thefrequency f₁ specified by FH sequence 1 is not disallowed so that thetransmitting node rules out FH sequence 0 during this timeslot andselects the allowed frequency f₁. As noted, the receiving node randomlyselect FH sequence 1 also, decoding on f₁ so that discovery is achieved.Had the receiving node randomly selected FH sequence 0, however,discovery would not have occurred. In an alternate version of thisembodiment, the receiving node would similarly avoid choosing FHsequence 0 during timeslot t₂ due to policy restrictions and wouldchoose to decode on the allowed frequency, f₄, specified by FH sequence1.

The flowchart in FIG. 9 shows the steps at a transmitting node accordingto the MFHS-PBSS embodiment. At step 901, for a timeslot, thefrequencies on which a beacon transmission would violate spectrum policyare determined. At step 902, all hopping sequences that during thattimeslot would select a frequency on which a beacon transmission wouldviolate spectrum policy are pruned from the set of B_(n) possiblehopping sequences. At step 903, one of the hopping sequences in thepruned subset is randomly selected. At step 904, a beacon is transmittedduring the timeslot at the frequency specified by the selected hoppingsequence.

The sequence of steps at a receiving node according the MFHS-PBSSembodiment is the same as is shown in FIG. 7 for the above-describedMFHS-RSS embodiment. For the alternate version of the MFHS-PBSSembodiment, for a timeslot, before attempting reception, the receivingnode randomly selects from among a subset of hopping sequences for whichspecified frequencies do not violate spectrum policy.

In the Multiple Frequency Hopping Sequences with Sequential EnergyDetection and Decoding (MFHS-SeEDD) embodiment, it is assumed that nodesenter transmit, receive, and idle discovery states in frames of Ltimeslots. Operation at the transmitting node is otherwise identical tothe MFHS-RSS embodiment described above. The transmitting node continuesto transmit on frequencies when allowed by spectrum policy according toa single selected hopping sequence that is used for each timeslot withinthe entire frame. The hopping sequence may differ, however, from frameto frame or may be constant across a plurality or all frames. Thereceiving node operation of this MFHS-SeEDD embodiment differs from theMFHS-RSS embodiment in that the frame is divided into L_(ED) and L_(DEC)timeslots, where L=L_(ED)+L_(DEC). In the first L_(ED) timeslots of aframe in which a receiving node is active, the receiving node performsenergy detection across M frequencies and L_(ED) timeslots to reduce thenumber of candidate hopping sequences from B_(n). For instance, if at aparticular timeslot, FH sequence i indicates beacon transmission onfrequency f, but no energy is detected on frequency f, the receivingnode assumes that beacon transmission is not occurring using FH sequencei. In this way, energy detection is assumed to be more sensitive thanthat of decoding and operates without error (e.g. if energy is detected,then it can be assumed that it originates from either a nodetransmitting a beacon or from an interference source). The number ofremaining candidate sequences after energy detection is defined to beB_(c), where B_(c) is at most equal to B_(n). In the last L_(DEC)timeslots of the frame, the receiving node randomly decodes according tothe remaining B_(c) sequences.

FIG. 10 shows an example of this embodiment where L=6 andL_(ED)=L_(DEC)=3. Since no energy is detected during t₀ at f₃ and noenergy is detected at f₈ during t₂, FH sequence 2 is eliminated sincethat hopping sequence specifies a beacon transmission at thosefrequencies during those timeslots. In the fourth timeslot, t₃, thereceiving node begins to decode randomly between the remaining twocandidate hopping sequences, FH sequence 0 and FH sequence 1.Alternatively, the receiving node may cycle through the remainingcandidate hopping sequences. As shown in FIG. 5, at the fourth timeslot,t₃, the receiving node selects to decode on the frequency, f₃, specifiedby FH sequence 1, but the beacon transmission was made on the frequency,f₅, specified by FH sequence 0. At the fifth timeslot, t₄, both of thefrequencies, f₁ and f₈, specified by FH sequence 0 and FH sequence 1,respectively, are disallowed by spectrum policy and discovery is notachieved. At the sixth timeslot, t₅, the beacon transmission is made onf₃ in FH sequence 0 and the receiving node also selects FH sequence 0and chooses to decode on f₃. Hence, barring poor RF conditions thatprevent successfully decoding of the beacon, discovery is achieved. Asin the previous embodiment, both the transmitting and receiving nodesmay have selected the same hopping sequence randomly or becauseselection was influenced by policy that disallowed the frequencyspecified by hopping sequence 1.

As noted, the steps at a transmitting node are identical to the MFHS-RSSembodiment as shown in the flowchart of FIG. 6, except that the hoppingsequence remains unchanged over each frame of L timeslots. The flowchartin FIG. 11 shows the steps at a receiving node in accordance with theMFHS-SeEDD node. At step 1101, for each timeslot in the first L_(ED)timeslots of a frame of L timeslots, the frequencies at which no energyis detected are determined. At step 1102, hopping sequences that wouldhave a beacon transmission during a timeslot and at a frequency forwhich no energy was detected are eliminated from the set of B_(n)possible hopping sequences. At step 1103, at each timeslot of theremaining L_(DEC) timeslots of a frame, an attempt is made to detect anddecode a beacon at a frequency specified by one of the remainingpossible hopping sequences. At step 1104, a determination is madewhether a beacon is detected. If yes, then at step 1105, discoverybetween the transmitting and receiving nodes is achieved. If a beacon isnot detected and decoded, then, at step 1106, discovery is not achieved.At step 1107, then, a determination is made whether this last timeslotwas the last timeslot in a frame. If yes, then, the flow returns to step1101 to process a next frame. If the last timeslot was not the lasttimeslot in a frame, the flow returns to step 1103 to choose a differenthopping sequence and associated frequency on which to attempt detectionand decoding for the next timeslot in the current frame.

In the Multiple Frequency Hopping Sequences with Simultaneous EnergyDetection and Decoding (MFHS-SiEDD) embodiment, energy detection anddecoding can be performed in the same timeslot in contrast with theabove-described MFHS-SeEDD embodiment where energy detection anddecoding are assumed to be performed in sequence, and where in eachtimeslot, a receiving node either detects energy or decodes. In theMFHS-SiEDD embodiment, the receiving node performs energy detection intimeslot S and uses the information derived from timeslot S in timeslotswith indices greater than S (e.g. to allow processing delay). As in theMFHS-SeEDD embodiment, the same hopping sequence is used throughout theframe consisting of L timeslots. In this MFHS-SiEDD embodiment, thereceiving node performs energy detection on successive timeslots, i, fori=1, . . . , L, within a frame, maintaining the number of remainingcandidate sequences, B_(c,i), for each successive timeslot such thatB_(c,i) is at most equal to B_(c,i-1). The receiving node randomly orcyclically selects a frequency from one of the remaining B_(c,i)candidate sequences to decode, where B_(c,i) may be based on informationgathered by energy detection in the current or previous frames.

FIG. 12 illustrates the operation of this method. In the first timeslot,t₀, of a frame consisting of L timeslots, the receiving node detects noenergy at frequency f₃, the frequency specified by hopping sequence 2.Thus, hopping sequence 2 is eliminated from further consideration asbeing the hopping sequence in use throughout this frame. During thisfirst timeslot, the receiving node selects to decode at frequency f₁,the frequency specified during this timeslot for hopping sequence 0.Since, however, as noted, the transmission was made at frequency f₅, thefrequency specified by frequency hopping sequence 2, beacon discovery isnot achieved. In the second timeslot, t₁, energy detection does notprovide further information and the receiving node randomly (or in acyclic manner) selects among the remaining possible hopping sequences 0and 1, and, in particular, selects frequency hopping sequence 0 and thefrequency, f₃, associated with that hopping sequence during thistimeslot. Since, however, the transmitting node and receiving nodes haveselected different hopping sequences, discovery is not achieved. In thethird timeslot, t₂, however, the receiving node selects f₁, thefrequency in this timeslot associated with frequency hopping sequence 1,which is the hopping sequence selected by the transmitting node in thisframe. Hence, barring poor RF conditions, neighbor discovery is achievedin this timeslot, t₂.

As in the MFHS-SeEDD embodiment described previously, the steps at atransmitting node for the MFHS-SiEDD are identical to the MFHS-RSSembodiment shown in the flowchart of FIG. 6, except that as in theMFHS-SeEDD embodiment, the hopping sequence remains unchanged over eachframe of L timeslots. FIG. 13 is a flowchart showing the steps at areceiving node in accordance with the MFHS-SiEDD mode. At step 1301, fora given timeslot within the frame of L timeslots, the frequency orfrequencies at which no energy is detected are determined. At step 1302,for each frequency at which no energy is detected during that timeslot,those hopping sequences for which use would have resulted in energy atsuch a frequency are eliminated from consideration. At step 1303, duringthat same timeslot, a hopping sequence is randomly or cyclicallyselected from the remaining possible hopping sequence and an attempt ismade to detect and decode a beacon at the frequency specified by thathopping sequence for that timeslot. At step 1304, a determination ismade whether the beacon was detected and decoded. If a beacon wassuccessfully detected and decoded, then, at step 1305, discovery isachieved. If a beacon was not successfully detected and decoded, then,at step 1306, discovery has not been achieved. At step 1307, adetermination is made whether the timeslot in which discovery was notachieved was the last timeslot in the frame. If it was the lasttimeslot, then discovery has not been achieved during the current frameand processing continues, at step 1308, with the first timeslot in thenext frame. The flow then returns to step 1301 for processing at firsttimeslot within the next frame. If, at step 1307, the determination ismade that the previously processed timeslot was not the last timeslot inthe frame, then processing moves, at step 1309, to the next timeslotwithin the current frame and the flow returns to step 1301 forprocessing at that next timeslot, with the hopping sequences eliminatedat step 1302 being accumulated with the hopping sequences previouslyeliminated during processing at a previous timeslot within the currentframe.

While the particular invention has been described with reference to theillustrative exemplary embodiments, this description is not meant to beconstrued in a limiting sense. It is understood that although thepresent invention has been described, various modifications of theillustrative embodiments, as well as additional embodiments of theinvention, will be apparent to one of ordinary skill in the art uponreference to this description without departing from the spirit of theinvention, as recited in the claims appended hereto. Those skilled inthe art will thus readily recognize that such various othermodifications, arrangements and methods can be made to the presentinvention without strictly following the exemplary embodimentsillustrated and described herein and without departing from the spiritand scope of the present invention. It is therefore contemplated thatthe appended claims will cover any such modifications or embodiments asfall within the true scope of the invention.

1. A method of neighbor discovery between a transmitting node and areceiving node in an ad hoc wireless network in which nodes communicatedirectly with each other, the method comprising: at a transmitting node:transmitting a beacon that changes frequency from timeslot-to-timeslotaccording to at least one predetermined frequency hopping sequence. 2.The method of claim 1 wherein a beacon is not transmitted during atimeslot when its transmission at the frequency determined by thefrequency hopping sequence during that timeslot would violate a spectrumpolicy.
 3. The method of claim 2 wherein the frequency hopping sequenceis a pseudo-random sequence.
 4. The method of claim 2 further comprisingselecting for a timeslot a frequency hopping sequence used to determinea beacon transmission frequency from among a plurality of differentpredetermined frequency hopping sequences.
 5. The method of claim 4wherein the frequency hopping sequence is randomly selected from amongthe plurality of different predetermined frequency hopping sequences. 6.The method of claim 4 wherein for a timeslot the frequency hoppingsequence is selected from among the plurality of different predeterminedfrequency hopping sequences that during that timeslot do not violate aspectrum policy.
 7. The method of claim 6 wherein the frequency hoppingsequence is randomly selected from among the plurality of differentpredetermined frequency hopping sequences that during that timeslot donot violate a spectrum policy.
 8. The method of claim 4 wherein thefrequency hopping sequence that is selected from among the plurality ofpredetermined frequency hopping sequences remains unchanged for eachtimeslot within a frame comprising a predetermined number of timeslots.9. The method of claim 8 wherein the frequency hopping sequence selectedfor each frame is randomly selected from among the plurality ofpredetermined frequency hopping sequences.
 10. A method of neighbordiscovery between a transmitting node and a receiving node in an ad hocnetwork in which nodes communicate directly with each other, the methodcomprising: at a receiving node: during a timeslot, attempting to detectand decode a beacon being transmitted by the transmitting node that ischanging frequency from timeslot-to-timeslot according to a frequencyhopping sequence, wherein neighbor discovery with the transmitting nodeis achieved when the beacon is successfully detected and decoded. 11.The method of claim 10 wherein a beacon is not transmitted during atimeslot when the frequency hopping sequence indicates a frequency thatwould violate spectrum policy during such timeslot,
 12. The method ofclaim 11 wherein the frequency hopping sequence is a pseudo-randomsequence.
 13. The method of claim 11 wherein at each timeslot at which abeacon is transmitted by the transmitting node, the frequency hoppingsequence is selected by the transmitting node from among a plurality ofdifferent predetermined frequency hopping sequences and during each suchtimeslot the receiving node selects one of the plurality of differentpredetermined frequency hopping sequences in attempting to detect anddecode the transmitted beacon.
 14. The method of claim 13 wherein ateach such timeslot the receiving node randomly selects one of theplurality of different predetermined frequency hopping sequences. 15.The method of claim 13 wherein at each such timeslot the receiving nodeselects one of the plurality of different predetermined frequencyhopping sequences from among those that indicate a beacon frequency thatdoes not violate spectrum policy during that timeslot.
 16. The method ofclaim 13 wherein the frequency hopping sequence selected by thetransmitting node from among the plurality of different frequencyhopping sequences remains unchanged for each timeslot within a framecomprising a predetermined number of timeslots, the receiving node:performing energy detection at each potential beacon frequency duringeach timeslot in a subset of the predetermined number of timeslots inthe frame; eliminating as a possible frequency hopping sequence used bythe transmitting node any hopping sequence which would have transmitteda beacon at a frequency at which no energy was detected during at leastone of the timeslots in the subset of timeslots in the frame; attemptingto detect and decode a beacon during a timeslot that is after the subsetof timeslots within the frame using a non-eliminated frequency hoppingsequence.
 17. The method of claim 13 wherein the frequency hoppingsequence selected by the transmitting node from among the plurality ofdifferent predetermined frequency hopping sequences remains unchangedfor each timeslot within a frame comprising a predetermined number oftimeslots, the receiving node: a) performing energy detection at eachpotential beacon frequency during at least one timeslot within theframe; b) eliminating as a possible frequency hopping sequence used bythe transmitting node any hopping sequence which would have transmitteda beacon at a frequency at which no energy was detected during the atleast one predetermined timeslot; c) attempting to detect and decode abeacon during the at least one timeslot within the frame using anon-eliminated frequency hopping sequence; and d) repeating steps a)through c) during successive other timeslots within the frame, whereinthe eliminated possible frequency hopping sequences are accumulated.